1. Technical Field
The invention relates to a process for recovering oil from a subterranean oil-bearing formation and more particularly to an oil recovery process utilizing a foam.
2. Background Information
The injection of displacement fluids into subterranean hydrocarbon-bearing formations to promote the production of hydrocarbons therefrom is well known. Water and various gases, in addition to more complex fluids, such as surfactant solutions and polymer solutions, are common displacement fluids utilized in both miscible and immiscible oil displacement floods.
The effectiveness of oil displacement floods is in part a function of the sweep efficiency of the displacement fluid. Sweep efficiency is substantially reduced by conformance problems in the formation and poor mobility characteristics of the displacement fluid. Conformance problems are generally characterized by the juxtaposition of high permeability and low permeability flow paths within a given formation. Conformance problems can be either fracture-type or matrix-type. Fracture-type conformance problems occur when there are simple fractures or fracture networks in communication with an injection and/or production well penetrating the formation. Matrix-type conformance problems occur when there are adjoining strata or regions of differing permeability within the formation.
When a displacement fluid is injected into a formation exhibiting conformance problems, the high permeability flow paths divert substantially all of the displacement fluid away from the low permeability flow paths. As a consequence, the displacement fluid does not sweep the low permeability flow paths and the sweep efficiency of the displacement fluid in the formation is poor.
Conformance problems can be corrected by a conformance improvement treatment (CIT) which effectively plugs or constricts the high permeability flow paths with a plugging material. By plugging or constricting formation flow paths having high permeability and low oil saturation, subsequently injected displacement fluid preferentially sweeps formation flow paths having low permeability and high oil saturation. Thus, the CIT improves the sweep efficiency of the displacement fluid, enables it to contact and displace more oil, and promotes increased incremental oil recovery.
At present, gels are commonly used as a CIT plugging material. Polyacrylamide crosslinked with chromium III has been found to form gels which are effective for most CIT's as set forth in U.S. Pat. No. 4,683,949 to Sydansk et al. U.S. Pat. No. 4,683,949 describes the effective use of crosslinked acrylamide gels specifically made for treatment of fracture-type and matrix-type conformance problems. It has now been found that in some cases CIT's employing conventional gels are prohibitively expensive due to relatively high chemical costs. For example, where high permeability flow paths extend far out into the formation away from an injection and/or production well, the flow paths can consume a tremendous volume of gel before an effective level of permeability reduction is obtained therein. A point is reached where the chemical requirements of the CIT are so high that the improved incremental oil recovery obtained by the treatment does not offset the CIT cost. Beyond this point, CIT's employing such gels are no longer cost effective and have no economic utility.
In such cases, a process is desirable which is more cost effective than conventional CIT processes using gels as the plugging material. More particularly, a CIT process is needed which has lower chemical costs than conventional gel treatments, yet which is at least as effective as conventional gel treatments. Furthermore, a CIT process is needed which is economical under conditions where conventional gel treatments are not, such as when the high permeability flow paths are pervasive throughout the formation.
Conventional gels have not only been found to be uneconomic in certain CIT's, but have also been found to perform unsatisfactorily in certain other plugging treatments similar to CIT's. For example, gels are typically ineffective for selectively plugging vertical fractures which extend from an oil-bearing production zone into a gas cap. Unless such fractures are plugged, the gas will be readily drawn into the production zone when the zone is produced which creates an undesirable gas coning condition. Unfortunately, because of the density of conventional gels and the vertical orientation of the fractures, the gels are extremely difficult to selectively place in the upper portions of such fractures where plugging would be most effective for prevention of gas coning. Gravitational forces cause the gels either to enter less problematic, lower-reaching fractures which do not communicate with the gas cap or to ineffectively settle only into the lower portions of the fractures which communicate with the gas cap. As such, an effective treatment is needed for sufficiently plugging the vertical fractures between the gas cap and the oil production zone to prevent gas coning at the production well bore.
In addition to conformance problems, poor mobility characteristics of the displacement fluid can also diminish sweep efficiency in the formation. Poor mobility characteristics can be countered by employing mobility control fluids known in the art which are typically more viscous than the displacement fluid. However, conventional mobility control fluids have been found to be ineffective for sweeping oil-bearing fractures and particularly fractures containing water and gravity-segregated oil or fractures in fluid communication with an aquifer.
Conventional mobility control fluids tend to preferentially sweep out water lying beneath the oil, which is denser than the oil, while leaving the oil behind. A process is needed which employs a more effective mobility control fluid for efficiently sweeping oil from a subterranean oil-bearing formation and particularly from oil-bearing fractures containing gravity-segregated oil or from oil-bearing fractures in fluid communication with an aquifer.